1. Field of the Invention
The present invention relates to the production of nonwoven webs of fibers with varying degrees of shrinkage.
2. Description of the Related Art
One of the primary reasons that certain polymers such as polyester are not more widely used in spunbond fabrics is that, unless the spinning speed of the fibers is greater than about 3500 meters per minute (MPM), the fabric will shrink by 50% or more during “post-spun” heating processes. Typically, “post-spun” heating processes include, but are not limited to, processes involving fabric bonding with hot calender rolls or other heat bonding devices. Shrinkage of the fibers occurs because the fibers are not crystallized, but instead are relatively amorphous, when produced at these low spinning speeds. Upon heating, such fibers contract or shrink as crystals are formed in an unconstrained state in the fibers, becoming thermally stable only when heated to their full crystallization temperatures (e.g., above about 120° C. for polyester) for a sufficient period of time.
To rectify this problem, spun bond draw jets and spunbond processes have been developed to accelerate the fibers to speeds above 3500 MPM during spinning, causing the spun fibers to become relatively crystalline during fiber formation. However, many conventional spun bond machines do not utilize such draw jets and processing capabilities, as these draw jets and processes tend to be complex and expensive to implement and operate.
Other types of nonwoven fibrous web forming processes experience similar heat shrinkage problems with polyester or other related polymers that remain relatively amorphous after fiber extrusion. In particular, polymers such as polyethylene terephthalate (PET) remain relatively amorphous when extruded using a meltblown process. A meltblown process differs from a spunbond process in that extruded polymer filaments, upon emerging from an extruder die, are immediately blown with a high velocity, heated medium (e.g., air) onto a suitable support surface. In contrast, extruded but substantially solidified filaments (e.g., solidified by a suitable quenching medium such as air) in a spunbond process are drawn and attenuated utilizing a suitable drawing unit (e.g., an aspirator or godet rolls) prior to being laid down on a support surface. Meltblown processes are typically utilized in forming fibers having diameters on a micron level, whereas spunbond processes are typically employed to produce fibers having normal textile dimensions.
Many attempts have been made to achieve heat stable and shrinkage controlled nonwoven fibrous webs of polyester utilizing a meltblown process. For example, U.S. Pat. Nos. 5,958,322 and 6,371,749 to Thompson et al., which are incorporated herein by reference in their entireties, disclose an apparatus and corresponding methods for making dimensionally stable nonwoven fibrous webs of polyester utilizing a meltblown process. In particular, the Thompson et al. patents disclose melt blowing polyester fibers and restraining a nonwoven web of the fibers on a tentering structure. The restrained fibers are then annealed or heatset in an oven to render the nonwoven web dimensionally stable up to at least the heatsetting temperature of the fibers.
The apparatus and corresponding methods described in the Thompson et al. patents require an excessive amount of fiber material to be wound around the tentering pins of the tentering structure in order to achieve a suitable restraining of the fibers during heat treatment. Depending upon the type of nonwoven fabric to be manufactured, the tentering process of Thompson et al. may result in a considerable amount of unnecessary or even undesirable fiber material in the fabric. This process would also not easily lend itself to operation at high fabric speeds as often used, e.g., in spunbond processes.
In addition, it is anticipated that a variety of unique nonwoven web products could be achieved by allowing fibers in a nonwoven web to shrink at varying degrees with respect to each other during web formation. However, previous attempts at controlling shrinkage of polymer fibers, such as PET fibers, during web formation have been primarily focused on limiting or preventing any shrinkage of the fibers. Thus, there has been very little effort in the art to produce nonwoven web products where shrinkage of fibers is encouraged to produce a desired physical result for the web product.
Accordingly, a process for manufacturing a variety of nonwoven webs of fibers with differing physical characteristics is desirable where the webs include fibers with varying degrees of shrinkage.